Pictures: Astronaut Scott Kelly's year living in space

NASA via AP

In this undated photo provided by NASA on Tuesday, March 1, 2016, astronaut Scott Kelly looks out the cupola of the International Space Station. Kelly closes the door Tuesday to an unprecedented year in space for NASA, flying back to the planet and loved ones he left behind last March.

In this Jan. 21, 2016 photo made available by NASA, one-year mission crew members Scott Kelly of NASA, left, and Mikhail Kornienko of Roscosmos celebrate their 300th consecutive day in space. By spending 340 days aboard the International Space Station, the astronauts will help scientists understand what happens to the human body while in microgravity for extreme lengths of time. Kelly is holding a zinnia plant grown in space as part of the Veggie experiment on the International Space Station. (NASA via AP)

NASA Astronaut Scott Kelly and Cosmonaut Mikhail Kornienko of Roscosmos spent 340 days aboard the International Space Station, the astronauts will help scientists understand what happens to the human body while in microgravity for extreme lengths of time.